Plot thickens in the hunt for a ninth planet

According to two theoretical physicists, the identical discoveries that sparked the search for a ninth planet may really be proof of a modified version of gravity that was first created to explain how galaxies rotate inside the solar system.

Researchers Katherine Brown, an associate professor of physics at Hamilton College, and Harsh Mathur, a professor of physics at Case Western Reserve University, made the claim after examining the impact the Milky Way galaxy would have on objects outside of our solar system—should the laws of gravity be governed by a theory called Modified Newtonian Dynamics, or MOND.

MOND suggests that the well-known rule of gravity by Isaac Newton is only partially true. That is, MOND permits a new gravitational behavior to take over when the gravitational acceleration predicted by Newton's law becomes modest enough.

Some scientists view MOND as a viable alternative to "dark matter," the term used by physicists to refer to a hypothetical kind of matter that would have gravitational effects but not emit light, because of its empirical effectiveness on galactic sizes.

"MOND is really good at explaining galactic-scale observations," Mathur stated, "but I hadn't expected that it would have noticeable effects on the outer solar system."

The Astronomical Journal just published their study.

A'striking' position

Mathur and Brown have previously investigated the impact of MOND on galactic dynamics. However, once astronomers revealed in 2016 that a few objects in the outer solar system had orbital abnormalities that may be explained by a ninth planet, they became interested in MOND's more local impacts.

Prior to now, historic discoveries have been made due to orbital peculiarities: Neptune was found by its gravitational pull on the orbits of nearby objects; Mercury's minute precession served as early evidence for Einstein's general theory of relativity; and most recently, astronomers have inferred the existence of a supermassive black hole at the center of our galaxy using orbital dynamics.

Brown noticed that the measurements that sparked the hunt for a ninth planet might not line up with MOND's predictions. "We wanted to see if the data that support the Planet Nine hypothesis would effectively rule out MOND," she stated.

Rather, Mathur and Brown discovered that MOND accurately forecasts the clustering that has been seen by astronomers. They suggest that certain objects in the outer solar system would have their orbits pulled into alignment with the galaxy's gravitational field over millions of years.

"The alignment was striking," said Mathur, when they projected the orbits of the particles from the Planet Nine dataset against the gravitational field of the galaxy.

The authors issue a warning, noting that the available dataset is tiny and that a variety of other explanations may turn out to be accurate. For instance, some astronomers have suggested that observational bias is to blame for the orbital irregularities.

"Regardless of the outcome," Brown stated, "this work highlights the potential for the outer solar system to serve as a laboratory for testing gravity and studying fundamental problems of physics."